STS 557 (17 alpha-cyanomethyl-17 beta-hydroxy-estra-4, 9-dien-3-one) or levonorgestrel was given orally as a single dose of 0.4 mg per animal. Treatment was performed immediately after the 18 h or 6 h mating period. Regarding the 18 h mating period, in 30 control cycles (20 animals) 11 pregnancies (36.7%) and in 20 cycles (16 animals) with levonorgestrel treatment 2 pregnancies (10.0%) were observed. In 80 cycles (31 animals) with STS 557 treatment 7 pregnancies occurred (8.7%). With regard to the 6 h mating period, in 30 control cycles (18 animals) 9 pregnancies (30%) and in 26 cycles (17 animals) with levonorgestrel administration 3 pregnancies were recorded (11.5%). In 50 cycles (21 animals) with postcoital administration of STS 557 only two pregnancies occurred (4%). Therefore, after the 6 h mating period STS 557 showed high interceptive activities in this non-human primate model. In comparison with controls, STS 557 did not decrease the postovulatory rise of plasma progesterone. This finding demonstrates that postcoital STS 557 in the chosen dosage does not suppress ovulation. Repeated STS 557 administration starting early in the follicular phase was without marked influence on cycle characteristics. On the other hand, treatment start in the middle or at the end of the cycle caused cycle lengthening and reduced progesterone levels in some animals.
Following oral and i.v. administration of [14 alpha, 15 alpha-3H]-STS 557 to beagle dogs, baboons, rats and female volunteers, plasma level courses of total radioactivity and STS 557, and radioactivity excretion in urine and feces have been investigated. Bioavailability of orally administered STS 557 was found to be 80--90% in man and beagle dog, 70--80% in baboon and rat. Concerning the systemic availability following oral administration of equivalent doses, the following order was established: beagle dog greater than man greater than baboon greater than rat. Equilibrium dialysis indicates species differences in plasma protein binding and a considerable part of STS 557 to be present in plasma unbound. STS 557 is rather rapidly eliminated from the plasma compartment of all species investigated with half lives less than or equal to 10 h. As an additional time parameter of pharmacokinetics the "mean residence time" was used. Urinary excretion of STS 557 metabolites is dominant in all species, including the rat. In contrast to the great part of STS 557 in plasma total radioactivity, only small amounts of unchanged STS 557 are excreted in urine. First results of current studies in rabbits are presented, too.
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